A recent survey showed that while many banks have service charges on interest-bearing and non-interest-bearing accounts
with balances that are falling below a set minimum, some banks provide virtually free checking regardless of the account balance.
(A) with balances that
are falling below
(B) whose balances fall below (C) whose balances fall below
that of (D) that have balances falling below
that of (E) that have balances that fall below
those ofC, D, and E can be eliminated because 'that of' or 'those of' doesn't make sense in this context -- this means there is a balance of a set minimum balance.
A is wrong because 'are falling' means this is is occurring in the present, but we're looking at a past survey here.
Answer should be B.